relation must evidently
always be smaller than unity, and must tend towards this value when
the mobility of the ions diminishes, that is to say, when the pressure
of the gas increases. The results obtained are in perfect accord with
this anticipation.

On the other hand, M. Langevin has succeeded, by following the
displacement of the ions between the parallel plates after the
ionisation produced by the radiation, in determining the absolute
values of the mobilities with great precision, and has thus clearly
placed in evidence the irregularity of the mobilities of the positive
and negative ions respectively. Their mass can be calculated when we
know, through experiments of this kind, the speed of the ions in a
given field, and on the other hand--as we can now estimate their
electric charge--the force which moves them. They evidently progress
more slowly the larger they are; and in the viscous medium constituted
by the gas, the displacement is effected at a speed sensibly
proportional to the motive power.

At the ordinary temperature these masses are relatively considerable,
and are greater for the positive than for the negative ions, that is
to say, they are about the order of some ten molecules. The ions,
therefore, seem to be formed by an agglomeration of neutral molecules
maintained round an electrified centre by electrostatic attraction. If
the temperature rises, the thermal agitation will become great enough
to prevent the molecules from remaining linked to the centre. By
measurements effected on the gases of flames, we arrive at very
different values of the masses from those found for ordinary ions, and
above all, very different ones for ions of contrary sign. The negative
ions have much more considerable velocities than the positive ones.
The latter also seem to be of the same size as atoms; and the
first-named must, consequently, be considered as very much smaller,
and probably about a thousand times less.

Thus, for the first time in science, the idea appears that the atom is
not the smallest fraction of matter to be considered. Fragments a
thousand times smaller may exist which possess, however, a negative
charge. These are the electrons, which other considerations will again
bring to our notice.

Sec. 3. HOW IONS ARE PRODUCED

It is very seldom that a gaseous mass does not contain a few ions.
They may have been formed from many causes, for although to give
precision to our studies, and to deal with a